The present invention relates to a medical device. More particularly, the invention relates to a guide wire and stent for insertion into a body lumen such as a blood vessel or a bile duct.
One technique that is used in the diagnosis and treatment of conditions such as heart disease involves inserting a guide wire to a target site, then inserting and passing a catheter or the like along the guide wire.
For example, in percutaneous coronary intervention (PCI), first a guide wire is advanced to, then made to traverse, the area of stenosis that is the target site while selecting the branches of the coronary artery under fluoroscopic guidance. Next, a dilatation catheter fitted at the distal end with a balloon is introduced into the body along the guide wire, and the balloon on the catheter is positioned at the stenosis. By then expanding the balloon and enlarging the lumen at the stenosis, blood flow is ensured. Conditions such as angina pectoris can be treated in this way.
Treatment can also be carried out by introducing a self-expanding stent to the target stenosis, and allowing the stent to self-expand at the stenosis.
Titanium-nickel alloys are sometimes used as the material making up such guide wires and stents. When a titanium-nickel alloy is used as the guide wire core, because the pushability and torque transmission of such alloys are inferior to those of materials such as stainless steel, the surface is sometimes coated with a resin to ensure good slideability.
To enhance the torque transmission and other properties, JP 3-264073 A discloses a catheter guide wire core that is a linear body made of a ferrous superelastic metal material, the core being distally tapered so as to become increasingly flexible toward the distal end. Examples of ferrous superelastic metal materials include Fe—Pt, Fe—Pd, Fe—Ni—Co—Ti, Fe—Ni—C, Fe—Mn—Si, Fe—Cr—Mn, Fe—Cr—Mn—Si, Fe—Cr—Ni—Mn—Si and Fe—Cr—Ni—Mn—Si—Co alloys. These are noted as being desirable because they have a high elasticity and are not readily prone to plastic deformation.
In connection with such ferrous superelastic metal materials, JP 03-257141 A, JP 2003-268501 A, JP 2000-17395 A, JP 09-176729 A, and Scripta Materialia 46, 471-475 describe Fe—Ni—Co—Al—C alloys, Fe—Ni—Al alloys, Fe—Ni—Si alloys, Fe—Mn—Si alloys and Fe—Pd alloys.
Good superelasticity is not achieved in conventional ferrous alloys because of, at the time of deformation, (a) the introduction of permanent strain such as dislocations, and (b) the formation of irreversible stress-induced lenticular martensite which does not exhibit shape memory effects. To avoid problems (a) and (b), it is effective to improve the strength of the matrix phase in ferrous shape-memory alloys; materials which are precipitation strengthened with an intermetallic compound are particularly effective. It is considerations such as these that have led to the disclosure of the foregoing ferrous alloys.
Although Ti—Ni alloys such as the above are sometimes used in guide wires and stents, the superelastic strain region for Ti—Ni alloys is at best about 8%; when a larger deformation is applied, the alloy undergoes plastic deformation, which is undesirable. It would be preferable to use a material having a broader superelastic strain region than Ti—Ni alloys as the guide wire and stent material.
In cases where a Ti—Ni alloy is used as the core material on the distal portion of a guide wire, a ferrous alloy is used as the core material on the proximal portion, and these core materials are welded together to form a single guide wire core, the fact that the Ti—Ni alloy is a different material which has, in particular, a poor weldability with ferrous alloys, imposes limitations on the materials to which it can be bonded and the bonding conditions. In medical devices for insertion and extended placement within the body in particular, the device must be manufactured while taking the greatest possible care to avoid the possibility of the weld failing within the body; hence the need for special bonding conditions, etc.
As noted above, in some cases where a Ti—Ni alloy is used as the guide wire core, the surface is coated with a resin. However, when a plastic material having a high melting point, such as a fluorocarbon resin, is used as the resin, the properties of the Ti—Ni alloy may change under the influence of the high temperature. The distal portion of the guide wire core, while slender, undergoes repeated bending and twisting. To be able to withstand such use, the resin coated onto the surface must have an improved peel resistance.
In addition, stents made of Ti—Ni alloys are sometimes of insufficient strength and durability. It is especially difficult to satisfy the requirements for strength and durability in stents used at placement sites where there is a lot of movement, such as the legs. Also, while it is preferable for a stent to be thin-walled, the strength in such cases decreases even further. Hence, thin-walled Ti—Ni alloy stents are often unable to withstand normal use.
Guide wires and stents made of Ti—Ni alloys also lack a good visibility under fluoroscopic imaging. To enable the insertion site and placement site to be verified under fluoroscopic imaging, a high-contrast member made of gold or the like must be bonded to the distal end of the guide wire and the ends of the stent.
The ferrous metal materials listed in JP 3-264073 A as preferable for use in guide wires are referred to as being “superelastic,” yet the amount of strain from which superelastic recovery is possible in such materials is in fact less than 1%, which hardly satisfies the properties required of a guide wire core.
Nor do JP 03-257141 A, JP 2003-268501 A and JP 2000-17395 A make any mention of such properties of practical importance as the amount of strain from which superelastic recovery is possible, the percent recovery, and the superelastic operating temperature ranges for Fe—Ni—Co—Al—C alloys, Fe—Ni—Al alloys and Fe—Ni—Si alloys.
Scripta Materialia 46, 471-475 reports on the superelasticity of Fe—Pd alloys containing unusually large amounts of costly palladium, but such alloys have an amount of strain from which superelastic recovery is possible of less than 1%, which is small, and thus cannot be regarded as having a good superelasticity. Moreover, these alloys are difficult to produce.
JP 09-176729 A mentions that Fe—Mn—Si alloys are nonmagnetic but, by utilizing a fcc/hcp transformation, exhibit a shape memory effect and superelasticity. However, there are limitations on the temperatures at which such Fe—Mn—Si alloys can be used because the superelasticity is achieved at temperatures higher than room temperature. Moreover, these alloys have a poor corrosion resistance and cold workability, complicated thermomechanical treatment is required to achieve further superelasticity, and there are problems with the manufacturability.